|I wouldn't say that adding chapter numbers would be a bad idea; rather, I think that it's simply much more advantageous to stick to volume numbers. The number of volumes is just more... "consistent" than the number of chapters, and is thus a more helpful detail. The number of chapters during serialization is not necessarily the same as in the tankobons. For example, volume 7 of Historie includes chapters 58-65 which are chapters 60-70 of the magazine serialization; not only is the numbering different, but 11 chapters were merged into just 8 chapters. You could probably reconcile this as:
Tankobon: 7 volumes (65 chapters)
Magazine: 70 chapters
But I think this has the potential to create more confusion.
Another argument for the current system that I can think of is that if we want readers to ultimately purchase legal copies, the number of volumes becomes more relevant than the number of chapters.